Complex structures in refractory bodies and methods of forming

ABSTRACT

A method of forming complex structures in a ceramic-, glass- or glass-ceramic-body microfluidic module is disclosed including the steps of providing at green-state refractory-material structure comprising least a portion of a body of a microfluidic module, providing a removeable insert formed of a carbon or of a carbonaceous material having an external surface comprising a negative surface of a desired surface to be formed in the microfluidic module, machining an opening in the green-state structure, positioning the insert in the opening, firing the green-state structure and the insert together, and after firing is complete, removing the insert. The insert is desirably a screw or screw shape, such that interior threads are formed thereby. The insert desirably comprises graphite, and the structure desirably comprises ceramic, desirably silicon carbide.

This application is a continuation of U.S. application Ser. No.14/361,829 filed on May 30, 2014, which claims the benefit of priorityunder 35 U.S.C. § 371 of International Application Serial No.PCT/US12/67015, filed on Nov. 29, 2012, which, in turn, claims thebenefit of priority of U.S. Provisional Application Ser. No. 61/564,917filed on Nov. 30, 2011 the content of which are relied upon andincorporated herein by reference in their entireties as if fully setforth below.

FIELD

The present disclosure relates generally to methods for forming complexstructures in refractory body microfluidic modules and to the modulesthus formed, more specifically, to methods for forming attachment systemstructures, such as screw threads, within a ceramic body microfluidicdevice in order to secure a connector, typically a metallic connectorfor fluidic interconnection.

BACKGROUND AND SUMMARY

Microreactors, or continuous flow reactors having channels micrometer—upto tens of millimeter-scale minimum dimensions, offer many advantagesover conventional batch reactors, including very significantimprovements in energy efficiency, reaction condition control, safety,reliability, productivity, scalability, and portability. In such amicroreactor, the chemical reactions take place continuously, generallyin confinement within such channels.

To avoid safety problems in strongly exothermic reactions, it isproposed in the art to perform these reactions in a microreactor. Inmicroreactors, these reactions are easier to control than inconventional batch reactors. In addition, it is possible in themicroreactor to realize reaction conditions which are not realizable forsafety reasons in a classical method in the laboratory or on theindustrial scale. Robustness and chemical resistance of the microreactorbody are therefore essentials when very corrosive compounds are employed(ie mineral acids or caustic solutions). Glass and ceramic materials(for example, Pyrex® glass, Aluminum oxide or Silicon Carbide), orglass-ceramics, are generally preferred for these conditions.

For glass, ceramic, or glass-ceramic microreactor, fluidicinterconnection fittings are generally attached to microreactor fluidicmodule body through a seal compression system which employs a polymero-ring tightened against an o-ring sealing location on the module body.The role of the o-ring is to ensure tightness but also to accommodatefor stresses (such as small amounts of tube flexion or expansionmismatch on heating) which can lead to the failure of the refractorymaterial of the fluidic module. Other techniques like brazing or weldingcan be applied, but these require very close matching of thecoefficients of thermal expansion of the materials involved.

One type of seal in current use employs an o-ring against flat portionof the microreactor body, forming an o-ring face seal, such as shown anddescribed for example, in FIG. 1 of EP1854543, a patent assigned to thepresent assignee. While the o-ring face seal shown in EP1854543 performswell enough under some conditions, there is a risk of the o-ring beingextruded through the small remaining clearance between the o-ring seatand the fluidic module face. Another disadvantage is the need forexternal structures encircling the fluidic module or reaching around theedges thereof in order to compress the o-ring against the module face.Such structures can increase the complexity and cost, and the footprintof a reactor which typically includes a plurality of fluidic modules.

One alternative fitting system is known in the art as “o-ring bossseal”. FIG. 1 (prior art) shows a cross-section of an instance a fluidicinterconnection system 10 employing an o-ring boss seal. A “boss” iscylindrical projection 22 on a structure 20, typically a cast or forgedmetal structure. The end 24 of the projection 22 is machined orotherwise formed to provide a flat, smooth surface 26 for sealing.Internal threads 28 are provided inside the bore of a fluidic port oropening 30 in the structure 20. A coupler 40 includes external threads42 to mate with the internal threads 28, a flat annular sealing surface44 to mate with smooth surface 26, and a gripping or engaging surface 46for threading the coupler 40 into the structure 20. When the coupler 40and the structure 20 are assembled together, an o-ring 50 is trapped inthe annular space closed off by the contact of surface 26 and surface44. Pressure from within the opening 30 merely wedges the o-ring moretightly between the coupler and a tapered inner surface 23 of theprojection 22. A tube 60 is inserted within a compressible sleeve 48,which is compressed against the tube 60 by a compression nut 49 toretain the tube 60.

A fitting system 10 like that of FIG. 1 is preferable to the o-ring faceseal system described above and in EP1854543 in at least two respects:(1) a pipe-thread style attachment is used, so no external fixtures arenecessary for attachment of the fixture and compression of the o-ring50, and (2) the o-ring 50, when in use, is seated into an enclosedcavity and cannot escape even at high pressure differentials. For atleast these reasons, it would be desirable to employ with a refractoryfluidic modules a fitting system similar to the fitting system 10 inFIG. 1, having an o-ring boss seal.

The main difficulty in employing a system similar to the fitting system10 in refractory materials like glass, ceramic and glass-ceramic is thatspecial tools and machinery are necessary to machine features withoutbreaking the materials. In general, grinding machines with diamond-facedtools are required, and the glass or the ceramic part must be cooledwith water to avoid excessive heat. In the case of very hard ceramics,such as SiC (silicon carbide), WC (tungsten carbide) or B₄C (boroncarbide), such a process is very expensive because the process very slowand the special and expensive tooling is quickly worn down. As aconsequence, the typical and preferred method to form these very hardmaterials is green-state machining—machining the ceramic in a green,unfired state. Unfortunately, the dimensional change on firing is toogreat for molding threads for fittings, since the shrinkage variation isabout 1-3%. Green-state machining to rough tolerances followed bypost-firing machining to final tolerance is possible, but still veryexpensive.

The present disclosure provides a method for forming complex structuresin refractory body-based microfluidic modules, in particular, forforming screw threads within a ceramic body or within a portion orcomponent of a ceramic body of a microfluidic device for mechanicalfastening, particularly in order to secure a coupler for fluidicinterconnection.

According to one embodiment of the disclosure, a method of formingcomplex structures in a ceramic-, glass- or glass-ceramic-bodymicrofluidic module is provided, including the steps of providing atgreen-state refractory-material structure comprising least a portion ofa body of a microfluidic module, providing a removeable insert formed ofa carbon or of a carbonaceous material having an external surfacecomprising a negative surface of a desired surface to be formed in themicrofluidic module, machining an opening in the green-state structure,positioning the insert in the opening, firing the green-state structureand the insert together, and after firing is complete, removing theinsert. The insert is desirably a screw or screw shape, such thatinterior threads are formed thereby. The insert desirably comprisesgraphite, and the structure desirably comprises ceramic, desirablysilicon carbide.

Certain variations and embodiments of the method of the presentdisclosure are described in the text below and with reference to thefigures, described in brief immediately below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of specific embodiments of thepresent disclosure can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 (prior art) is an elevational cross-section of a fluidicinterconnection fitting system employing an o-ring boss seal;

FIG. 2 is a flow diagram of an embodiment of steps in the method of thepresent disclosure;

FIG. 3 is an elevational view of a temporary insert according to anembodiment of the present disclosure in the form of a screw;

FIG. 4 is a cross-sectional view of a portion of a green-staterefractory body having an opening machined therein to receive thetemporary insert;

FIG. 5 is a cross-sectional view of the portion of a green-staterefractory body of FIG. 4 with the temporary insert of FIG. 3 positionedwithin the opening therein; and

FIG. 6 is a cross-sectional view of the green-state refractory body ofFIGS. 4 and 5, but now in a post-fired state after firing of thegreen-state body and removal of the temporary insert, showing thepresence of complex structures, in the form of internal threads, formedby the firing of the green-state refractory body with the temporaryinsert positioned within the opening.

DETAILED DESCRIPTION

Creating complex or fine structures in engineering ceramics is costly.Even when the majority of the work is carried out in the green state,there is still a need for diamond machining to remove the majority ofthe surface defects and imperfections that could act as critical flawsin service. Further, when the structures to be formed are standard screwthreads, the geometry and dimensional tolerances of typical metallicmachine screw threads are very tight (100 micrometers or less). Sinteredceramic structures often undergo shrinkage of as high as 15-20% duringfiring of the structure. Therefore, it is very difficult to form threadswith required tolerances out of a sintered ceramic product withoutrequiring significant post-machining. For these reasons engineers todaycommonly employ clamping and bolting arrangements for mechanical joiningof glass or ceramic with other solid parts.

The present disclosure provides a manufacturing process which allows thesimple screwing of a metallic threaded fluidic coupler, such as the(prior art) coupler described above and shown in FIG. 1, directly intothe body of a ceramic microreactor. Other complex structures may also beformed by the present method, the basic steps of which are diagrammed inthe process flow diagram of FIG. 2.

The method 100 as represented in FIG. 2 includes a step 70 of providinga green-state refractory-material structure comprising least a portionof a body of a microfluidic module. The refractory-material is selectedfrom glass, ceramic, glass-ceramic and mixtures or combinations ofthese, including filled sinterable materials wherein glass, ceramic,glass-ceramic and mixtures or combinations of these comprise thesinterable components of the material.

The method 100 further comprises a step 72 of providing a removeableinsert formed of carbon or of a carbonaceous material. The externalsurface of the insert comprises a negative surface of a desired surfaceto be formed in the ceramic-body microfluidic module. The negativesurface may be formed by machining from a carbon block, for example.

The method 100 further comprises a step 74 of machining an opening inthe green-state structure and positioning the insert in the opening. Theopening is preferably made so as to give just-sufficient clearance foreasily positioning the insert in the opening, such as 100 micrometerclearance, for example, although it may be larger or smaller if needed.It is desirable that the open volume between the negative surface of theinsert and the interior surface of the opening, or the portion thereofto be formed, is sufficiently small, and the shrinkage of thegreen-state structure, on firing and conversion to post-fired state, issufficiently large, such that the open volume is closed curing thefiring process, such that the interior surface of the opening, orportion thereof to be formed, conforms to the surface of the insert.

The method 100 further comprises step 76, of positioning the insert inthe opening, step 78, firing the green-state structure and the inserttogether, and step 80, after consolidation of the structure, removingthe insert. If the insert is not a relatively dense carbon material, theinsert may be removed by oxidation. Nevertheless, the insert must besufficiently durable during the firing process such that it retains itsshape through sufficient consolidation of the structure for formation ofthe desired positive surface on the interior surface of the opening. Ifthe insert is comprised of a relatively dense carbon material, theinsert may be removed by mechanical means (simply being unscrewed, inthe case of a screw-form insert) or by oxidizing the insert, such as byhigh temperature oxidation in air or oxygen, or by other knowntechniques.

The principle application envisioned for this method is shown in part inFIGS. 3-6. A graphite screw 120, as shown in FIG. 3, may be employed asa mirror image insert with a negative surface 122 (the screw threads).To use the graphite screw 120 in the present method, an opening 130 isformed, such as by machining (drilling) in a green-state structure 140.In this embodiment, the opening 130 includes a wider portion 132 toreceive the insert in the form of the graphite screw 120. FIG. 5 showsthe opening 130 with the insert in the form of a graphite screw 120positioned therein. The outer diameter of the screw threads 122 isalmost the same as the inner diameter of the wider portion 132 of theopening 130, with desirably about 100 micrometers clearance. Firing thencauses the green-state structure 140 to shrink, and, after removal ofthe screw 120, interior threads 150 remain, formed on the inner surfaceof the opening 130 of the fired structure 140 f, as shown in FIG. 6. Thethread features can be molded at any sides of the planar components(top, bottom and even the edge) of a microfluidic module, provided thatthe size and/or thickness of the plate is big enough to embed thegraphite screw.

The thermal expansion of the graphite material should be close enough tothe ceramic or the glass in order to prevent excessive residual stressduring cooling of the parts. The insert material should also be chosencarefully to prevent undesired chemical reactions between the screw andthe ceramic or the glass body. Any reactions can change the nature ofthe material locally, and produce inferior properties. Graphite is anexample material which is desirable with glass and Silicides (eg, SiC,Si3N4, MoSi2). Graphite material also has the advantage of being a veryelastic material (Young's modulus of about 10 MPa) and thus limitsstresses. Different variations of graphite material also offer a largerange of thermal expansion coefficients (from 5 t 80×10⁻⁷/° C.) andgraphite withstands very high temperature when in an inert atmosphere.Graphite grade 2020 available from Mersen can work with SiC and Si3N4ceramics or with low expansion Borosilicate glasses (such as Pyrex®glass).

The present method is of particular interest for use with siliconcarbide ceramic since its properties are very attractive formicroreactor application and the post-machining of such a hard ceramicis unaffordable. A green part of SiC can be obtained by cold pressingand the opening 130 can be machined simply with conventional cuttingtool. The graphite screw is then be seated into a the opening.Afterwards, the green-state body is fired at 2100° C. in a non-oxidizingatmosphere to prevent formation of oxides of silicon in preferencesilicon carbide, and to prevent burning of the graphite insert. Afterdensification, the structure is desirably be cooled, such as to roomtemperature, then finally oxidized in air at 1000° C. to remove thegraphite insert.

The methods disclosed herein and the devices produced thereby aregenerally useful in performing any process that involves mixing,separation, extraction, crystallization, precipitation, or otherwiseprocessing fluids or mixtures of fluids, including multiphase mixturesof fluids—and including fluids or mixtures of fluids includingmultiphase mixtures of fluids that also contain solids—within amicrostructure. The processing may include a physical process, achemical reaction defined as a process that results in theinterconversion of organic, inorganic, or both organic and inorganicspecies, a biochemical process, or any other form of processing. Thefollowing non-limiting list of reactions may be performed with thedisclosed methods and/or devices: oxidation; reduction; substitution;elimination; addition; ligand exchange; metal exchange; and ionexchange. More specifically, reactions of any of the followingnon-limiting list may be performed with the disclosed methods and/ordevices: polymerisation; alkylation; dealkylation; nitration;peroxidation; sulfoxidation; epoxidation; ammoxidation; hydrogenation;dehydrogenation; organometallic reactions; precious metalchemistry/homogeneous catalyst reactions; carbonylation;thiocarbonylation; alkoxylation; halogenation; dehydrohalogenation;dehalogenation; hydroformylation; carboxylation; decarboxylation;amination; arylation; peptide coupling; aldol condensation;cyclocondensation; dehydrocyclization; esterification; amidation;heterocyclic synthesis; dehydration; alcoholysis; hydrolysis;ammonolysis; etherification; enzymatic synthesis; ketalization;saponification; isomerisation; quaternization; formylation; phasetransfer reactions; silylations; nitrile synthesis; phosphorylation;ozonolysis; azide chemistry; metathesis; hydrosilylation; couplingreactions; and enzymatic reactions.

It is noted that terms like “preferably,” “commonly,” and “typically,”when utilized herein, are not utilized to limit the scope of the claimedinvention or to imply that certain features are critical, essential, oreven important to the structure or function of the claimed invention.Rather, these terms are merely intended to identify particular aspectsof an embodiment of the present disclosure or to emphasize alternativeor additional features that may or may not be utilized in a particularembodiment of the present disclosure.

Having described the subject matter of the present disclosure in detailand by reference to specific embodiments thereof, it will be apparentthat modifications and variations are possible without departing fromthe scope of the invention defined in the appended claims. Morespecifically, although some aspects of the present disclosure areidentified herein as preferred or particularly advantageous, it iscontemplated that the present disclosure is not necessarily limited tothese aspects.

It is noted that one or more of the following claims utilize the term“wherein” as a transitional phrase. For the purposes of defining thepresent invention, it is noted that this term is introduced in theclaims as an open-ended transitional phrase that is used to introduce arecitation of a series of characteristics of the structure and should beinterpreted in like manner as the more commonly used open-ended preambleterm “comprising.”

What is claimed is:
 1. A method of forming complex structures in aceramic-, glass- or glass-ceramic-body, the method comprising the stepsof: providing a green-state refractory-material structure; providing aremoveable insert formed of a carbon or of a carbonaceous material, anexternal surface of the insert comprising a negative surface of adesired surface to be formed; machining an opening in the green-statestructure, the opening being of sufficient size to leave an open volumebetween the external surface of the insert and an interior surface ofthe opening when the insert is positioned in the opening; positioningthe insert in the opening; firing the green-state structure and theinsert together; during firing, reducing or closing the open volume; andafter firing is complete, removing the insert.
 2. The method accordingto claim 1 wherein an open volume between the negative surface of theinsert and an inside surface of the opening is sufficiently small, andwherein a shrinkage of the green-state structure upon firing issufficiently large, such that the open volume is closed during thefiring process and such that said surface of the opening conforms to thenegative surface of the insert.
 3. The method according to claim 1wherein firing comprises firing in an inert atmosphere.
 4. The methodaccording to claim 1, wherein removing comprises oxidation of theinsert.
 5. The method according to claim 1, wherein the insert is ascrew and wherein the complex structure comprises interior screwthreads.
 6. The method according to claim 1, wherein the step ofproviding a green-state structure comprises providing a structurecomprising one or more of glass, glass-ceramic and ceramic.
 7. Themethod according to claim 1, wherein the step of providing a green-statestructure comprises providing a structure comprising a ceramic.
 8. Themethod according to claim 1, wherein the step of providing a green-statestructure comprises providing a structure comprising silicon carbide.